Lithium-ion battery is a green battery of high average output voltage, large output power, low self-discharge, no memory effect, wide operating temperature range (between −20° C.˜60° C.), excellent cycling performance, high charge-discharge efficiency, long service life and contains non-toxic and hazardous substances. It relies on the lithium-ions' round-trip of embedding and de-embedding between a negative electrode and a positive electrode to complete battery charging and discharging operations.
While electric vehicles have many advantages when compared to conventional vehicles, but there are still many problems when the lithium-ion battery pack is applied to the electric vehicles, the most significant problem to be resolved is the group technology of the lithium-ion battery, the most involved problem is the energy density of the battery pack, which directly affects the mileage of the electric vehicles.
Energy density of the lithium-ion battery pack is mainly determined by the framework of the battery pack, usually the lithium-ion battery cells are collected in parallel first, then the parallel modules are collected in series, and the connection of tabs of the battery cell requires the use of the current collector sheet, in the prior art, the parallel connection of the battery cells is welding the taps of the battery cell directly on the current collector sheet, while the series connection is using wires to connect the current collector sheets; but combination of this battery module has the following disadvantages: 1. when the battery is in the process of operation, it is easy to loose the welding point between the wires and the current collector sheet while under vibration or shocking, the results includes pseudo soldering and desoldering; 2. for there are other wires for sampling such as sampling lines in existence, then the adding of the wires, makes the internal battery more complicated or messy, and furthermore it takes up too much real estate or space.
Patent with application No. CN201010142205.5, titled “the battery pack” discloses a battery pack composed of a plurality of soft-package battery cells, which includes connecting means that connect the two batteries, the batteries and the connecting means are connected together by wires, thus the space of the battery pack is occupied via a large foot print or greatly, the energy density is reduced, and the welding between the wires and the connecting means is spot welding, it tends to become loose by shocking in the operation process, and thus results in pseudo soldering and desoldering.
Again, among the products that the Tesla company puts on the market, the battery module of the product with maximum energy density used by the ModelS has the size of length 2.7 m, width 1.5 m, thickness 0.1 m to 0.18 m, is composed of 7600 batteries of 18650, for a 18650 battery cell, the battery shell is approximately 0.02 dm, the electrode sheet's length is 0.65-0.05 after removing the top and bottom members of the battery, therefore the volume of the electrode sheets of the 18650 battery cell is 3.14*((0.18−0.004)/2)2*(0.6)=0.0146 liters, the volume of the electrode sheets used in the batteries of the entire vehicle is 0.0146*7600=111 liters, the volume of the battery module is 27*15*1+3*15*0.8=441 L. In summary, the volume ratio that the electrode sheets occupied the battery pack (Electrode Volume Ratio hereinafter is abbreviated as EVR) is 111/441=25.2%, under the same conditions that the other materials are the same, the higher proportion the electrode sheets take, the higher energy density of the battery pack is, the lower the contrary, 25.2% of EVR for the electric vehicle has greater constraints to the improvement of the volume energy density, it is not conducive for the arrangement of the power battery to make the mileage of power battery electric vehicles with the indicators of the traditional vehicle in order to meet the needs of customers (the single fuel mileage of the traditional vehicle is almost 600 kilometers, while the longest mileage of Tesla is only about 400 kilometers through a single charge).
Moreover what the battery pack uses is the cylindrical battery cell, the cylindrical battery cell can be welded to the connecting pieces only by spot welding, and because the contact area is small between the poles of the battery cell and the connecting pieces, so this reduces the current through capacity, also increases the heat, and combining such number of cylindrical battery cells into the battery pack, the heat generated will be very great or significant, the design for heat-dissipation will be more complicated, also more energy will be wasted, these further reduce the effective use of the energy in the overall lithium-ion battery pack.